1. Field of the Invention
The present invention relates to a control technique for an internal combustion engine with a variable valve mechanism of an automatic stop-and-restart vehicle.
2. Description of the Related Art
There have been developed automatic stop-and-restart vehicles in which an engine (an internal combustion engine) is automatically stopped when the vehicle stops running so as to increase the fuel economy. With the automatic stop-and-restart vehicle, the engine is automatically stopped when a predetermined stop condition is met in which the vehicle speed is 0 with the engine running at idle state, for example. In addition, the engine is automatically restarted when a predetermined restart condition is met in which the application of brakes is released after the engine has been automatically stopped.
On the other hand, there have been developed engines with a variable valve mechanism in which a lift amount of an inlet valve of the engine is varied. In an engine with the variable valve mechanism described above, in order to ensure the startability, the setting of the engine can be changed to a setting in which the actual compression ratio becomes relatively high when the engine is started or to a setting in which the actual compression ratio becomes relatively low by suppressing the amount of intake air in association with a throttle valve when the engine is running at idle state.
However, when the engine is set so that the actual compression ratio becomes high in the way described above, although the startability is increased, the torque fluctuation becomes large. Therefore, there is caused a problem that vibrations and striking noise of the starter become large when the engine is stopped and restarted. In the automatic stop-and-restart vehicle, noise and the like are desirably suppressed to as low a level as possible so as not to be sensed by the occupants when the engine is automatically stopped and restarted.
Then, in an automatic stop-and-restart vehicle including an engine with a variable valve mechanism, there has been developed a technique to control the variable valve mechanism so as to suppress an actual compression ratio when the engine is automatically stopped or restarted (Patent Document 1).
In Patent Document 1 above, the actual compression ratio is reduced by increasing a lift amount of an inlet valve when the engine is automatically stopped or restarted so as to retard a valve closing timing thereof during a compression stroke.
However, even in the event that a throttle valve is set to be closed while the engine is automatically being stopped, a negative pressure between the throttle valve and the inlet valve varies with time. When the engine is restarted in that state, since the lift amount is being set on the increased side, the amount of intake air is affected largely by a negative pressure working then, and the revolving speed and torque of the engine have not yet been stabilized, there being caused a fear that the occupants feel a sensation of physical disorder.
[Patent Document 1] Japanese Patent Publication Number 2000-34913